It is known that the biological treatment, for example of water, consists in degrading the organic impurities by the action of a purifying biomass that is free or fixed and that contains a variety of micro-organisms such as bacteria, yeasts, protozoa, metazoa etc. In the method using free biomass, such as activated sludge, high concentration of the various species of micro-organisms which have little settling capacity, is impossible to achieve inasmuch as the concentration of the biomass is obtained by settling. The method is therefore limited as regards the load applicable in terms of BOD (biological oxygen demand) and COD (chemical oxygen demand). In a system with a fixed biomass, the concentration of the biomass (with the bacteria) is achieved by making the bacteria cling to a carrier medium. The settling capacity is then no longer the essential criterion and this technique possesses a purification potential far greater than that of standard methods.
Among the most efficient methods based on the principle of purification with fixed biomass, we may cite notably those patented and developed by the Applicant, in a single upflow reactor of a granular bed constituted by two zones having different granulometry and different biological characteristics (French patents No. 76 21246 published under No. 2 358 362; No. 78 30282 published under No. 2 439 749; No. 86 13675 published under No. 2 604 990).
In the so-called free biomass techniques, reference will be made here especially to methods using fluidized beds wherein the material used as the biofilter consists of products with a density of less than 1 such as, for example, expanded polymers, according to processes which are now in the public domain (French patent No. 1 363 510 dated 1963; U.K. patent No. 1 034 076 dated 1962), various variant embodiments of which have led to numerous invention patents (French patents Nos. 330 652, 2 406 664, 2 538 800; U.S. Pat. No. 4,256,573; Japanese patent No. 58-153 590 etc.).
The use of these floating materials and of fluidized granular beds is promising in itself but entails a number of difficulties and frequently shows drawbacks. For example, if materials heavier than water (such as sand or similar materials) are fluidized, then a considerable input of energy is needed for the pumping of the liquid and it is difficult to control the keeping of the materials inside the reactor. To overcome this drawback of energy consumption, it has been proposed to use a fluidized bed with light materials, having lower density than water, with an insufflation of air at the base of the bed but with a supply of descending water (U.S. Pat. No. 4,256,573 and Japanese patent No. 58 153590 referred to here above). However, from a certain downflow speed of the water, the air bubbles are trapped within the material or else carried along by the liquid flow and it is not possible to aerate the reactor properly.
The difficulties of the prior art have been overcome by the development of a system as disclosed in Applicant's application EP0504065, wherein in a single reactor or biological filter with ascending co-currents of water and gas, the filtration means and bacteria support medium used is a fixed bed of particles having lower density than water with a density of 35 to 65 kg/m3. Especially preferred is the use of expanded polystyrene balls with a granulometric size ranging from 2 mm to 6 mm
The reactor of EP0504065 comprises from bottom to top: a zone for the expansion and removal of sludge of the media and settling of loosened sludge; at least one air injection device; a zone of filtering material constituted by a layer of the above-mentioned light particles, a ceiling made of concrete or other perforated material and, finally, at the top of the reactor, a washing water storage zone at the tip of which there is provision for the removal of the treated effluent.
Another reactor developed by the Applicants is disclosed in EP0347296, wherein the reactor is equipped with a lower fluidized bed and an upper fixed bed for filtration. The particles in the beds are composed of expanded particles having a density of less than 1. The particles of the fixed bed are both smaller and lighter than those of the fluidized bed.
In this system also a single reactor or biological filter with ascending co-currents of water and oxygenated gas is used. For the combination of the two aforementioned superposed beds, the process according to EP0347296 implements materials that are lighter than water but whose properties of granulometry, density, bed height vary, so that on the one hand a fluidization of the lower bed during the injection of the oxygenated gas without appreciable perturbation of the upper bed, and, on the other hand, an “automatic” reclassification of the two layers or beds during the phase in which the light materials expand when washed with a countercurrent is achieved.
At rest, these two layers of materials lighter than water stick together because of their different densities. This classification is maintained while the filter is washed with the counter current. When air is introduced into the base of the filter by a diffusion device, the air and water mixture passing through the materials has a similar density to the particles in the aforementioned lower layer. The lower bed in this case is fluidized by the ascending movement of the oxygenated gas bubbles, which causes an intensive exchange between the gases, the water to be treated and the “biofilm” which adheres to the particles of the bed.
For the lower fluidized bed, the granulometry can vary from 3 to 15 mm, the volume mass is generally between 300 and 800 g/l and the height of the bed ranges from 0.2 to 2 meters depending on the type of reactor used; in the upper fixed bed, the average diameter of the light particles is from 1 to 10 mm, while the volume mass varies from 20 to 100 g/l and the height can vary from 0.5 to 3 meters. Finally, in the case of the aforementioned variation, the upper layer over mounting the upper bed comprises particles from 3 to 20 mm in size, having a volume mass of 10 to 50 g/l and a height or thickness of 0.10 to 0.50 meters.
The particles of light materials that can be used as a filtering medium/bacterial support are expanded plastic materials, closed-cell materials from polyolefins, polystyrene, synthetic rubber polymers and copolymers, etc.; light mineral materials such as clay or expanded shale, or cellulosic products such as wood particles, for example. The granulates of these materials can be in various forms, such as, advantageously: balls, cylindrical pods, etc. In practice, for the effective execution of the process, it is important for the densities of the light particles used within the context of the invention to be increasingly low as we move from the lower layer (fluidized bed) to the upper layer, and then to the aforementioned support layer. For example, the density ranges can respectively be: 0.5 to 0.8 (fluidized bed); 0.3 to 0.1 (fixed bed) and 0.005 to 0.08 (upper support bed).
In another application FR2741872 Applicant discloses another water treatment reactor in which a fixed bed and a fluidized bed are combined. The reactor has a first filtration zone of rigid PVC material with fixed 3D structure and a second filtration zone packed with filling material, like for example expanded polystyrene balls, that has a lower density than water and is fixed. Since a frequent problem of such reactors is the particle loss during backwashing in counter current this reactor provides for a space between the two zones that allows expansion of the fixed bed of the second filtration zone during washing. Means of oxygen injection are placed in this space. Oxygen is therefore only injected above the first zone, which stays anoxic. The air only goes into the second zone. In this reactor two different zones are combined, one for denitrification, the other for nitrification.
The particles as used in these bioreactors do not provide for any protected surface area for growth of the biofilm on the particles, since the particles used are small spheric particles. The biofilm can therefore grow only on the surface of the spherical particles, which is not protected from any damage that may be caused by collision of the spherical particles.
In contrast to that carriers as disclosed in EP0750591 are large and provide a large surface for the biofilm, protected against wear, without the efficiency of the process being heavily reduced by a still greater oxygen limitation of the biofilm than in available small carriers.
The large carrier elements of EP0750591 have a structure similar to a turbine wheel with radial inside walls that are interconnected by outer rings and forming several axial passages. The large area of inner surfaces of the carrier is thus protected against wear against the surfaces of other carriers. Furthermore the flow passages allow good flow through of water. Other suitable carriers are described in EP 1340720 and EP05785314.
The carrier elements of EP0750591 have a density close to the density of water so that the carriers with the biofilm are kept suspended and moving in the water in the reactor. This avoids that water stays stationary in the carriers and ensures that air can pass through the inner passages of the carrier.
Applicants of present invention have combined the advantages of the setup of their previous reactors with the advantages of the type of carriers as for example disclosed in EP0750591, EP 1340720 and EP05785314 to provide for an improved method of water purification that shows highly increased yield. At the same time the object of present invention is to provide for a reactor that is not increased in volume while providing for a higher yield.
The solution provided is an improved reactor and a method for biological purification of wastewater according to present invention as further described here below.